Four-wheel drive vehicles can be categorized into part-time four wheel drive vehicles that basically operate in a two-wheel drive state and can switch to a four-wheel drive state if necessary, and full-time four wheel drive vehicles that include center differentials disposed between the front wheels and the rear wheels and constantly operate in the four-wheel drive state (for example, refer to Japanese Unexamined Patent Application Publication No. 2000-247159). Four-wheel drive vehicles that include transfer devices and operate in both part-time and full-time types or in multiple different control modes (traveling modes) have been put to practical use.
For example, Japanese Unexamined Patent Application Publication No. 2010-149850 discloses a four-wheel drive vehicle that includes a mode selector switch switchable among six traveling modes. The four-wheel drive vehicle operates in a normal traveling mode at start-up of an off-road guidance ECU and then switches to one of the traveling modes selected with the mode selector switch. The driving arrangement (position) recommended for the selected traveling mode is displayed on a guidance screen for presentation of the operating method of the vehicle to a driver. Such presentation can promote the setting of the recommended driving arrangement of the driving system of the vehicle suitable for the selected traveling mode.
The technique described in Japanese Unexamined Patent Application Publication No. 2010-149850 provides guidance to a driver on a recommended driving arrangement (drive mode of the vehicle) suitable for the traveling mode preliminarily selected by the driver. For other four-wheel drive vehicles, a driver preliminarily selects the drive mode of the vehicle and then selects the traveling mode. The drive modes of a vehicle include a two-wheel drive (2WD) mode, a full-time four-wheel drive (full-time 4WD) mode, and a direct-connection four-wheel drive (direct-connection 4WD) mode. The traveling modes include a normal traveling mode and off-road traveling modes suitable for off-road driving.
The off-road traveling modes are selected when the driving force should be certainly transmitted to a rough road for driving, for example, in deep snow, sandy ground, or muddy ground. The off-road traveling modes can usually be set in the direct-connection 4WD mode. The selector for the traveling mode (off-road traveling mode), however, is installed on a position such as an instrument panel that is easy to be operated by the driver and readily accessible by a person in the passenger seat. Thus, in some cases, the selector may be improperly operated in a drive mode other than the direct-connection 4WD mode or by the person's prank in the passenger seat. Switching the traveling mode to a mode inappropriate for the drive mode due to improper operation of the selector may lower the traveling performance of the four-wheel drive vehicle.
The transfer device includes a mechanism including multiple gears and coupling sleeves. The arrangement of this mechanism is controlled to switch the drive mode. The transfer device actually enters a drive mode selected by a driver in a predetermined time after the selection. Thus, the drive mode selected by the driver does not correspond to the arrangement of the transfer device for a predetermined time.
Some transfer devices have an arrangement corresponding to the direct-connection 4WD mode (direct-connection 4WD arrangement) between the arrangement corresponding to the 2WD mode (2WD arrangement) and the arrangement corresponding to the full-time 4WD mode (full-time 4WD arrangement). On the other hand, a drive-mode selector operated by a driver is provided to switch the different modes in an increasing order of driving force, i.e., the 2WD mode, the full-time 4WD mode, and the direct-connection 4WD mode, selected in this order.
If the transfer device has the configuration described above, the transfer device shifts from the 2WD arrangement to the full-time 4WD arrangement via the direct-connection 4WD arrangement in response to the driver operating the drive-mode selector from the 2WD mode to the full-time 4WD mode. This further prolongs the time during which the drive mode selected by the driver does not correspond to the arrangement of the transfer device.
Thus, even if the driver properly operates the selector for the traveling mode (off-road traveling mode), the actual arrangement of the transfer device may not correspond to the selected drive mode, and a traveling mode unsuitable for the drive mode may be selected. This may cause a reduction in the traveling performance of the four-wheel drive vehicle. The control modes of various drive operation assist devices installed in the vehicle are switched in accordance with the traveling mode. Thus, setting of an appropriate traveling mode is important for the safety of the vehicle.